This application claims priority of DE 198 51 743.2, filed Nov. 10, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system for positioning and moving machine elements, particularly workpiece grippers or receiving devices, for the transport or handling of workpieces.
The machining of workpieces frequently requires transporting them between several machining stations, for example, between press stages. As a rule, the workpieces must be moved along a defined path. For this purpose, the workpieces are picked up by means of corresponding gripping or receiving devices and are moved from one station to another. In each station, the gripping or receiving devices release the workpiece and, optionally by way of a parked position, move back to the starting position in the machining station which is preceding in the machining sequence. The whole path covered by the gripping devices is called a transfer curve. For this purpose, it is known, for example, from German Patent document DE 4418417 A1 to provide, in the case of a transfer press, two mutually parallel transfer rails 3, on which travelling carriages are in each case disposed in a longitudinally displaceable manner. The transfer rails extend along a transport path which leads through all press stations. A cross traverse extends from a travelling carriage on one transfer rail to a travelling carriage on the other transfer rail. The travelling carriages are connected with driving devices by means of which these can be adjusted and driven in a targeted manner along the travelling rails. The transfer rails are coupled with stroke devices, which are capable of synchronously lifting and lowering the two transfer rails. By the superimposition of the lifting and lowering movement of the transfer rails and of the transfer movement of the travelling carriages, the transfer curve of the cross traverses is formed.
In addition, it is known, for example, from German Patent Document DE 3218450 A1 to transport sheet metal pieces by means of a gripper which can be swivelled about a vertical axis. The gripper has a telescope arm with a gripper head and, for the transport of pieces, swivels about a circular arc situated in a horizontal plane.
By means of such a device, curves can be generated which are situated in a horizontal plane; that is, the gripper head is moved within a horizontal plane. The drive of the gripper, which includes a swivel drive and a telescope drive, has to apply the force required for accelerating the sheet metal piece and for braking it. Because of the lateral arrangement of the swivelling axis, the swivel drive and the telescope drive are, however, free of weights.
If a curve situated in a vertical plane is to be travelled, that is, a curve which has a horizontal transport component as well as a vertical lifting and lowering component, the drives, which cause the lifting and lowering, must, additionally to the force required for accelerating the workpiece, apply a force for overcoming the force of its weight. This weight is also a function of the time and the position; that is, a gripper which is moving while it is empty generates a clearly lower weight than a gripper loaded by means of a workpiece. The drives must be designed for the maximal driving power to be transmitted to the gripper. As a result, they are correspondingly heavy and have a high weight.
It is an object of the invention to be able to use drives which are as light as possible.
This object is achieved in the case of a system which has the characteristics of claim 1.
According to the invention, in addition to the driving force, a compensation force is supplied to a drive and force transmission device, which is used for moving and positioning a carrier device, for compensating the force of the weight applied to the carrier device. This is advantageous particularly in the case of devices which have drives moved along by other axes. The force of the weight emanates from the carrier device, from the workpiece held at the carrier device and possibly from parts of the drive and force transmission device. If now, for example, the weight of the carrier device and of the moved-along part of the drive and force transmission device is compensated, only the weight of the workpiece will still be applied to the drive causing the vertical movement. The torque to be applied by the drive is correspondingly lower and so is the required power. In addition, the friction in a transmission arranged between the drive and the carrier device may possibly be reduced.
When the workpiece is deposited, the compensation force in the vertical direction additionally introduced into the drive and force transmission device causes a complete weight compensation. In addition, a slight overcompensation, for example, by half the workpiece load may also be possible. However, the drive which can apply the same power for the path section of the carrier device without any workpiece as in a path section with a workpiece, in this case, is within its power limit. The compensation of approximately half the amount of the weight of the workpieces and the almost complete compensation of the weight resulting from the parts of the positioning device itself which would act upon the drive without the compensation, results in a reduction of the maximal forces acting upon the drive and in a time-related uniformity of the amounts of the forces. The drive on the whole can be designed for lower power and therefore becomes smaller and lighter.
The force generating device can be arranged to be acting in parallel to the mechanical driving device of the positioning system. A flow of force therefore exists from one or several driving motors to the carrier device. The path of the flow of force is formed by suitable transmissions, which may also include lever mechanisms. The compensation force of the force generating device is supplied directly to the carrier device or to a suitable point in the force transmission path from the drive or drives to the carrier device. As required, the force generating device may also be coupled directly to the driving device.
Particularly if applied directly to the carrier device, the force generating device can generate a time-constant compensation force and transmit it to the carrier device. The force of the force generating device may be adjustable in order to permit an adaptation to different workpiece weights.
Furthermore, it is possible to construct the force generating device such that it can generate a time-variable compensation force. This is possible, for example, by means of pneumatic cylinders, whose internal pressure is controlled as a function of the path and/or the time by suitable solenoids by way of a control unit. As a result, it is possible, beyond the above-mentioned approximation-type compensation of the workpiece weight, to vary the compensation force for the transport stroke and the empty stroke, so that the weight component still occurring at the drive is almost completely compensated. In addition, it is therefore possible to compensate the weight in the case of those positioning systems, in the case of which the component of the weight acting upon the drive changes as a function of the position. This may occur, for example, in the case of swivel arms which can be swivelled about one or several horizontal axes. When the swivel arm is swivelled out of its vertical line, the weight applied to the swivel arm causes a swivelling moment whose preceding sign depends on the swivelling direction. When the swivel arm is in its vertical position, no swivelling moment will occur. With an increasing inclination of the arm and, if the arm can be telescoped, with its increasing length, the swivelling moment becomes larger. The compensation, which ideally is to be carried out as an upwardly acting vertical force at the carrier device, can in many cases not be carried out easily from above. If the weight on the swivel arm, on its bearing or a moved-along telescope drive is compensated, it is advantageous for the compensating force to be controllable.
The weight compensation is conceivable in the case of different drive kinematics. The positioning system may, for example, have one or two swivel arms which extend, for example, in parallel to one another and which, at the end side, are connected by a cross traverse with the workpiece gripping devices, for example, suction devices. In an advantageous embodiment, the swivel arms may be swivellably disposed at their respective other end on a slide block which is in each case vertically displaceably disposed at a press stand or at another frame device. The adjustment of the swivelling position of the swivel arm can be carried out by two guide rods which meet in the center on the swivel arm at an articulation and which, in turn, are disposed in an articulated manner on the slide block. These slide blocks are driven and therefore define the vertical position and the swivelling position of the swivel arm. The force generating device may be a pneumatic device which acts upon a fourth slide block displaceably disposed in the vertical direction. The fourth slide block is again connected with the swivel arm by way of a guide rod. The force of the weight applied to the carrier can therefore be compensated relatively precisely for all swivelling positions within a working range of the positioning system, without the requirement that the force generating device would have to adjust the introduced force.
Furthermore, it is possible to spatially define the horizontal axis of the swivel arm and to drive the swivel arm, for example, by way of a linkage or a rotary drive. As the result of an additional telescoping capacity of the swivel arm, it is again possible to travel through almost arbitrary transfer curves. The rotary drive as well as the telescope drive may be separately connected with a force generating device which generates a compensation force which differs according to the swivelling position and the telescoping position.
Instead of taking place at a fixed axis or a linear guide, the guide of the supported end of the swivel arm can also be formed by another guide, for example, by a cycloidal guide.